The present invention relates to isolated polynucleotide molecules useful for analyzing alloantigen phenotypes, to peptides encoded by these molecules, and to the diagnostic and therapeutic uses thereof relating to human platelet Pl.sup.A polymorphism, including a method for typing blood cell and platelet membrane glycoproteins which entails an analysis of amplified cDNA encoded by platelet and red blood cell mRNA.
Blood obtained from different individuals has been found to have different antigenic and immune properties, to the extent that antibodies in the blood of one person may react with antigens on red blood cells or platelets in the blood of another individual. These antigens are often found on membrane glycoproteins present on the surface of the cell membranes. These membrane glycoprotein antigens can induce the production of antibodies against them when they are introduced as foreign proteins in transfused blood or in fetal blood. Human platelets and red blood cells contain dozens of identifiable membrane glycoprotein constituents, only some of which have been well characterized.
Membrane glycoproteins which induce antibody production in the same species are called "alloantigens." Alloantigens have been characterized for both red blood cells and platelets. Recognized classes of red blood cell and platelet alloantigens have been described, over the past 30 years, based on observations of antibody reactions occurring when patients have been exposed to blood from other individuals. The lack of sequenceable antigen protein and clonable antigen-encoding mRNA has prevented molecular characterization of the different alleles coding for many clinically important alloantigens.
One system of alloantigens, consisting of the platelet Pl.sup.A1 and Pl.sup.A2 alloantigens, are carried by the human platelet membrane glycoprotein IIb-IIIa (GPIIb-GPIIIa) complex, which mediates platelet aggregation by providing functional receptors for fibrinogen on platelet surfaces. See Phillips et al., Blood 71: 831-43 (1988). GPIIb and GPIIIa are known to bear a number of clinically important, alloantigenic determinants which are responsible for eliciting an immune response in two well-described clinical syndromes, post-transfusion purpura (PTP) and neonatal alloimmune thrombocytopenia (NATP). See Kunicki & Newman in CURRENT STUDIES IN HEMATOLOGY AND BLOOD TRANSFUSION 18-32 (1986); Aster in ADVANCES IN IMMUNOLOGY AND BONE MARROW TRANSPLANTATION 103-118 (1984).
The alloantigen system most frequently implicated in these disorders is the Pl.sup.A alloantigen system. There are two serologically defined, but molecularly undefined, allelic forms of the Pl.sup.A alloantigen, designated "Pl.sup.A1 " and "Pl.sup.A2," which are thought to be expression products of the GPIIIa gene. Kunicki & Newman, id., at 18-32. The gene frequencies for these two alleles have been calculated to be 85% for Al and 15% for A2, see Shulman et al., J. Clin. Invest 40: 1597-620 (1961). Since 98% of the population carries the Pl.sup.A1 antigen, individuals who are Pl.sup.A2 homozygotes are at risk of producing anti-Pl.sup.A1 antibodies against paternally-inherited Pl.sup.A1 antigens present on fetal platelets, and are most likely to develop PTP following blood transfusion.
Determination of the amino acid sequence variations that are presumably responsible for forming the relevant epitopes of red blood cell and platelet alloantigens has been achieved in only a few instances, due largely to the formidable difficulties in obtaining protein-sequence information from those often large glycoproteins. For example, the amino-acid sequence variation responsible for the relevant epitopes has not yet been reported for either the Pl.sup.A1 or Pl.sup.A2 forms of the 100 kilodalton (kd) GPIIIa molecule. In this regard, there is considerable data to support the notion that platelets and red blood cells, since they are anucleate cells, possess only vestigial amounts of protein-synthetic capability, although specific protein biosynthesis has been demonstrated in platelets, see Plow, Thromb. Haemostasis 42: 666-78 (1979); Kieffer et al, Eur. J. Biochem. 164: 189-95 (1987); Belloc et al, Biochim. Biophys. Acta. 925:218-25 (1987), and in newly formed red cells (reticulocytes). Thus, it has not been considered feasible to obtain platelet or red blood cell cDNA libraries, thereby to analyze alloantigen-encoding sequences and determine the molecular basis of the alloantigen phenotypes.